This invention generally is directed to a method for manufacturing weld wire having reduced inclusions and specifically at a method for producing superalloy weld wire for use in repair of superalloy articles for which inclusion control is important.
Superalloy weld wire used for welding superalloy components is available commercially, but has been found prone to oxide inclusions. It was determined that these oxide inclusions were directly related to the method of producing the wire. Oxide inclusions are undesirable because they are transferred from the weld wire to the work piece during welding operations. Unless the welder is very skilled, the inclusions are incorporated into the weld. The non-metallic inclusion in the weld is an undesirable discontinuity in an otherwise continuous metallic matrix, and, in the worst case, can lead to a crack.
Weld wire is manufactured by casting superalloy material into cast ingot in the form of pipe with an equiaxed grain structure having a diameter of about one inch. The casting operation produces standard and well-known casting defects that have been transferred to the weld wire, which, up until now, have been accepted as inherent in the process of producing the weld wire. Casting involves melting metal of the desired composition and placing it into a mold of preselected shape where heat is withdrawn. For weld wire, the superalloy metal is cast into pipe using metal molds. The solidification process progresses as heat is extracted from the mold surface. As the heat is extracted, small crystals begin to form at the mold wall and grow as a substantially equiaxed-grain structure. As the metal transforms from liquid to solid at the solidification interface, shrinkage results from the solid metal occupying less volume than the liquid metal. This shrinkage is not a problem, so long as molten metal is available to supply metal to the area of shrinkage. The cylindrical molds currently used to produce the castings for the weld wire have a solidification interface that advances from the inner surface of the mold into the molten metal as heat is extracted outwardly through the mold wall. If the solidification interface progresses radially completely across the cylindrical pipe-like molds used for weld wire before the molten metal below it has solidified, then feed metal is prevented from reaching this portion of the casting. Thus, as this molten metal freezes, there is no path for additional molten metal to feed the region, and shrinkage cavities result. It has been found that undesirable inclusions, such as oxide inclusions and other impurities, float to the surface of molten metal during solidification into ingot where they can be removed. When the feed path for molten metal is closed, not only is there no path for supplying additional molten metal to compensate for shrinkage, but there is no path to allow the inclusions to float to the surface of the mold. The result is that these areas of shrinkage, which when extensive are referred to as pipe, or which appear as localized areas of porosity, also act as collector areas for oxide inclusions, dirt or other impurities formed during the solidification process. Once the porosity and the associated non-metallic inclusions are formed within the casting, there is no practical way to remove them. The casting is then removed from the mold, typically by grinding.
The pipe typically is cut into standard lengths of 25 inches and xe2x80x9ccanned.xe2x80x9dThere is no restriction on the length into which the pipe is cut, the length being limited as a function of the size and capability of the extrusion press. Canning entails placing a plurality of one inch diameter bars in a can with an inert oxide, such as alumina oxide or silica, and sealing the can shut by vacuum welding or electron beam welding. The xe2x80x9ccannedxe2x80x9d pipe is extruded into rod having a diameter of xc2xc inch, and the canning is removed from the surface. Defects, inclusions and dimensional variations are then ground from the exterior of the rod. Of course, since only inclusions on the exterior are removed, internal inclusions are extruded along the axial length of the xc2xc inch rod. The extrusion cycle is then repeated by drawing standard 25 inch lengths of xc2xc inch canned wire to the required diameter of about 0.042 inches to about 0.045 inches, the inclusions having been further extruded in the axial direction.
Because most oxides that result in inclusions formed during the solidification process melt at temperatures significantly higher than the melting temperatures of the superalloy metal, the oxides do not dissociate during the welding. In fact, certain superalloy compositions, such as Rene"" 142, include hafnium, which, upon oxidation forms hafnia, one of the most stable oxides known. Only the most skilled of welders can prevent this oxide from being entrapped in a weldment, such as repaired tip of a superalloy turbine blade.
What is needed is a method of producing superalloy weld wire that has reduced non-metallic inclusions, so that weldments produced by such weld wire result in cleaner welds containing fewer non-metallic inclusions, thereby resulting in fewer rework cycles and lower repair costs.
Improvements in manufacturing technology and materials are the keys to increased performance and reduced costs for many articles. As an example, continuing and often interrelated improvements in processes and materials have resulted in reduced costs and reduced repair cycles for parts used in aircraft gas turbine engines. The present invention is one such improvement. A method for producing weld wire having low inclusion content is set forth. The method includes casting a molten metal alloy of a preselected composition. The casting is accomplished in a mold having at least one ingot cavity that is shaped in a rod size of a preselected diameter. The molten metal filling the rod-shaped ingot can then be directionally solidified by conventional directional solidification technology. The directional solidification permits control of the solid/liquid metal interface, so that heat constantly is withdrawn away from the advancing interface as columnar grains grow by advancing into the liquid interface. This method of solidification substantially eliminates the formation of shrinkage cavities and porosity. Also, because the columnar grains grow by advancing into the molten metal, impurities such as oxides and dirt are pushed away from the advancing grains and are not trapped within the ingot as it solidifies. The impurities in the form of oxides, dirt and other non-metallic inclusions gather in the last portion of the ingot to solidify, so that they can readily be removed.
After the rod-shaped castings have solidified and the region containing the impurities has been removed, they can optionally be arranged into a plurality of preselected equal lengths. The rods are then placed into a container of malleable metal. Because the container is disposable, it is typically made from an inexpensive material such as steel. The remaining volume of the container is occupied by an inert material that will not affect the rods during subsequent mechanical processing. The filled container is then sealed by placing a cap on it, which is welded in place.
The filled container is then extruded to a preselected diameter in a conventional manner for manufacturing weld wire. The weld wire is removed from the container, typically by destroying the extruded container by grinding.
An advantage of the present invention is that weld wire made in accordance with the present invention has improved cleanliness, the impurities associated with oxide inclusions, shrinkage, dirt and other non-metallic inclusions being substantially reduced or eliminated. Because these impurities are always transferred to the weld pool during welding operations, there is a reduced tendency for the structures being welded to incorporate these impurities. These impurities, when present in sufficient amounts, can form defects and even lead to cracking. Thus, welds made using weld wire of the present invention do not require exceptional skill to accomplish and require less repairs.
Another advantage of the present invention is that weld wire made in accordance with the present invention has a more homogenous chemical composition. This eliminates problems associated with segregation, such as cracking due to localized embrittlement induced by mechanical working without a homogenization heat treatment. Also, a more homogenous composition is desirable as weld metal is deposited on the workpiece.
Still another advantage of weld wire made in accordance with the present invention is that it can be made more cheaply and with a higher yield than prior art weld wire. Even though the casting and directional solidification methods are more costly, the costs are more than offset since the material can be cast to a preselected rod size, allowing at least one extrusion operation to be eliminated. In addition, the grinding operation associated with removal of the can after extruding typically results in grinding of at least some of the weld wire. Since at least one grinding operation is eliminated, yield is improved.
Finally, the rods are directionally solidified so that the columnar grains are aligned along the rod axis, which is also the most ductile direction. Because this is also the direction of the extrusion operation, the wire is easier to extrude into longer lengths with less breakage during the extrusion process.